A problem is that the elastomeric nature of the sealing rings mounted around a piston in a cylinder together with the very high pressures which have to be sustained in these assemblies means that the elastomer may be extruded in the axial direction of the piston. Also, if its sealing effect is to be sufficient it has to be forced radially against the wall of the cylinder; against which it slides, hard enough in order to ensure an efficient seal. This means that there may be wear and rubbing at that wall so that the working surface of the sealing ring is rolled off as well as perhaps suffering extrusion.
It has been proposed to provide a seal ring assembly having an elastomeric sealing ring and also harder rings on each side in which there is interaction between the sealing ring and the harder ring in the sense that the sealing ring being elastomeric "energises" overhanging portions of harder rings which are placed on each axial side of it. This is an advantageous situation as it helps hold both the sealing and these harder rings from displacement and assists the bearing action of the harder rings.
However, these harder rings tend to lift radially outwardly in use and this can open up a gap under their radially inner edges into which material of the seal ring can enter: by lessening radially outward pressure on the harder rings this tendency, which can give rise to distortion and "nibbling" of the sealing ring, can be eliminated or lessened.